it needs to be serviced by a person who is qualified in unvented cylinders (not all are).

The Megaflo is an excellent product, fairly simple and very reliable unless neglected. It has various safety devices to prevent it going over pressure, so it ought to be inspected annually, for example when you have your boiler serviced. Unqualified repairs or alterations might make it less safe.

It might be that your pressure release valve has failed, or the immersion heater thermostat is allowing it to overheat, or the expansion vessel is faulty.

If hot water is squirting down the side of your building, you are wasting money and energy heating it, so turn off the heater; and you will be damaging your building, by making the wall wet, so turn off the water. You will have to use a kettle until it is fixed.